This invention relates to material for broadband-selected absorption and reflection of electromagnetic waves for all frequencies below optical (hereinafter referred to as RF) using dilute concentrations of metallic particles suspended in a lightweight dielectric for such applications as radar cross section suppressors, microwave heat exchangers, plasma generators, transformation of RF energy into thermal optical energy, shields for RF discharge devices, etc. , and lightweight RF broadband reflecting material for such applications as radar reflectors, broadband antennae with prescribed or variable gain pattern, optically transparent RF shields, EMI shields and RF filters.
Such absorptive material is useful in antenna-communication links (low elevation angles) , RF microwave laboratory insulation, EMI absorptive shields. Such reflective material is useful for satellite antennas, communication links, cable television, shielding electronic computers, computer games, microwave ovens, commercial RF microwave laboratory shielding and in high RF energy discharge technology in industrial research and development. Additional utilization may be in frequency-band sensitive radar beacon reflectors, EMI filters, selective absorbers in solar heat exchange devices and RF transparent thermal insulators. Still other uses may be suggested to one skilled in the art from the following description of the invention which exploits the efficient RF absorbing and reflecting properties of dilute concentrations of small, suitably shaped metal particles and metal coated dielectric particles suspended or sustained in a low loss dielectric material. The terms metal and metallic are used interchangeably to denote materials with high electrical conductivity.
Methods of synthesizing dielectric materials with dilutely distributed metallic particles for the achievement of effective RF band-selective absorption and reflection have been limited to semiempirical trial and error recipes, tested by repeated measurement. They have the limitations of expense in the time consuming and materially costly repetitive testing and successive modification of the dielectric ingredients to achieve results that suffer from excessive mass or weight requirements and are constrained by narrow band performance, and highly complex circuitry or microcircuitry.
The internal electric field, and hence the electric moment of an RF irradiated particle, is derived from the perturbation solution to Green's theorem integral equation depicting the depolarizing effect of the induced surface charge and the power dissipation due to the volume current. The resultant internal field is thus the depolarized incident field within an attenuation depth from the particle surface, referred to hereinafter as skin depth. The depolarizing factor derives naturally from the integral formulation, as the internal solid angle subtended by the surface normal component of the incident electric field. The efficiency of absorbers is then characterized by their depolarizing factors, conductivities, and ac permeabilities for ferromagnetic materials.
Through the conventional Lorentz-Lorenz formulation of the composite permittivity of dilute distributions of particle dipole classes, the coefficients of power reflection and absorption of the synthetic dielectric medium are established. The penetration lengths, mass requirements and mean constitutent particle dimensions and conductivities are described or prescribed in parametric form for high absorption, and its complement, high reflection within broad frequency bands. The volume fraction parameter used in the description refers to the volume fraction of metallic conducting material.